Commercially available personal computer systems include a printed circuit board known as the motherboard. The motherboard contains the central processing unit for the personal computer, general control circuits, input and output connectors and also includes card slots or edge connectors for attaching various expansion cards.
Conventional personal computer systems also include a system chassis. The system chassis provides a mechanical structure for the personal computer. Various components of a personal computer system, such as a floppy disk drive, a hard disk drive, an optical disk drive, a cooling fan, a power supply and the motherboard are all attached to the system chassis.
Generally, the system chassis forms an enclosure, the interior surface of which is metallic to provide governmentally required electromagnetic shielding. This shielding prevents electromagnetic energy produced by the personal computer system from escaping the chassis where it can interfere with other electronic devices located near the personal computer system. Such shielding may also improve the performance of the personal computer system by preventing environmental electromagnetic energy from entering the enclosure and interfering with circuits of the personal computer system located therein.
The motherboard is typically mounted to the system chassis using standoffs. A conventional standoff is a piece of sheet metal folded into a U-shape which is inserted into a rectangular opening cut into a base of the chassis. A threaded hole is formed in the base of the U-shape and a screw is passed through a aperture in the motherboard and secured to the standoff through the threaded hole. Other conventional standoffs include pressed in studs or may be formed by stamping and folding the system chassis.
The standoffs prevent electrical contact between this electromagnetic shielding and exposed electrically active circuitry on the motherboard, such as printed traces, solder bonds, jumper wires and the like. The standoffs prevent such contact by positioning the motherboard at a fixed distance from the chassis. In addition, the insertion into and removal of expansion cards from the motherboard can apply significant forces to the motherboard which may tend to dislodge the motherboard from the chassis. Therefore, the standoffs must be able to resist such forces.
In addition to mechanically positioning the motherboard, the standoffs and screws provide an electrically conductive path between the system chassis and a ground plane or ground node on the motherboard. A solid, low impedance connection to ground is important to ensure the reliability of the computer system. For example, a low impedance ground connection helps suppress electromagnetic interference. For this purpose, a portion of the motherboard surrounding each mounting aperture is plated with an electrically conductive material. Less commonly, the interior of each mounting aperture may also be plated. An electrical connection is then established between the plating material and the standoff when the screw is tightened.
Personal computers are typically manufactured on a large scale wherein small reductions in per-unit cost can aggregate to a significant amount of savings or increased profit. Conventional methods for mounting a motherboard to the system chassis generally utilize from five to eight screws for each motherboard. It is well known that using screws adds time and expense to a manufacturing process. For example, the motherboard must be properly aligned with the screw holes, any lock washers must be applied to the screws, then the screws are typically applied and loosely threaded into the accepting screw holes, and finally, once all the screws are in place, the several screws are tightened. This repetitive and recursive process is difficult to automate and thus requires significant human labor. In addition, screws can add to product quality problems as incorrect installation can cause damage to the motherboard or screws may be forgotten. Therefore, a technique is needed for mounting the motherboard of a personal computer system to the chassis of the personal computer system that uses a minimum number of screws. Such a technique should provide a solid electrical ground connection between the motherboard and the chassis and should provide a secure mechanical attachment of the motherboard to the chassis that is able to resist forces associated with insertion and removal of expansion cards.
Further, the input and output connectors which are mounted to the motherboard must be accessible from the exterior of the personal computer system. An opening is therefore provided in the chassis for such access. This opening may, however, reduce the effectiveness of the electromagnetic shielding. Conventional personal computer systems provide a gasket to fill gaps between the input and output connectors and the system chassis or between the input and output connectors and a trim panel of a computer case that is exterior to the chassis. To form an effective electromagnetic shield, however, this gasket must firmly contact the chassis or trim panel and the input and output connectors. Therefore, what is further needed is a means for ensuring firm contact between a gasket surrounding the input and output connectors and the chassis or trim panel of a personal computer system.